Participation of (eta(3)-allyl)ruthenium(II) complexes in C-C bond formation and C-C bond cleavage. A theoretical study

Coupling reactions of formaldehyde with RUBr(eta (3)-C3H5)(CO)(3) (R1), [RU (eta (3)-C3H5)(HCHO)(CO)(3)](+) (I2), and RuBr(eta (3)-C3H5)(HCHO)(CO)(2) ( I3) were theoretically investigated with ab initio MP2-MP4(SDQ), CCSD(T), a nd DFT(B3LYP) methods. In R1, the coupling reaction takes place through two transition states, as follows: coordination of formaldehyde with the ruthe nium(II) center occurs through the first transition state, to afford an (et a (1)-allyl)ruthenium(II) formaldehyde complex, RuBr(eta (1)-C3H5)(HCHO)(CO )(3), as an intermediate, and then C-C bond formation between the eta (1)-a llyl ligand and formaldehyde takes place through the second transition stat e, to afford RuBr(OCH2CH2CH=CH2)(CO)(3). The activation energy (E-a) is 19. 9 (12.0) kcal/mol for the first transition state and 12.5 (5.4) kcal/mol fo r the second transition state, where the values given without parentheses a re E-a values calculated with the MP4(SDQ) method and the values in parenth eses are those calculated with the DFT method. In I2 and I3, the coupling r eaction proceeds through one transition state, to afford [Ru(OCH2CH2CH=CH2) (CO)(3)](+) and RuBr(OCH2- CH2CH=CH2)(CO)(2) with considerably larger E-a v alues of 50.7 (30.6) and 34.8 (32.0) kcal/mol, respectively. Even in I2, ho wever, the allyl-aldehyde coupling reaction easily occurs through two trans ition states like that of R1, when one more formaldehyde molecule coordinat es with the ruthenium(II) center; the E-a value is 10.5 (4.6) kcal/mol for the first transition state and 13.6 (6.7) kcal/mol for the second transitio n state. In this case, one formaldehyde molecule plays the role of a specta tor ligand and the second formaldehyde undergoes a coupling reaction with t he allyl ligand. From these results, it should be concluded that the allyl- aldehyde coupling reaction proceeds easily in the coordinatively saturated (eta (3)-allyl)ruthenium(II) complex and that the coordinatively unsaturate d (eta (3)-allyl)ruthenium(II) complex becomes reactive when two molecules of formaldehyde coordinate with the ruthenium(II) center. IRC calculation o f the allyl-aldehyde coupling reaction of R1 clearly shows that the C-C bon d formation between the eta (1)-allyl ligand and formaldehyde occurs after the second transition state concomitantly with the bond alternation in the eta (1)-allyl ligand. Electron redistribution in the reaction indicates tha t the allyl-aldehyde coupling reaction is understood in terms of electrophi lic attack of formaldehyde to the allyl ligand. Reverse C-C bond cleavage p roceeds with a moderate E-a value of 16.6 (13.5) kcal/mol in [Ru(OCH2CH2CH= CH2)(CO)(3)](+) to afford [Ru(eta (3)-C3H5)(HCHO)(CO)(3)](+), with a simila r E-a value of 19.6 (11.1) kcal/mol in RuBr(OCH2CH2CH=CH2)(CO)(3) to afford RUBr(eta (3)-C3H5)(CO)(3) + HCHO, and with a considerably large E-a value of 27.0 (26.8) kcal/mol in RuBr(OCH2CH2-CH=CH2)(CO)(2) to afford RuBr(eta ( 3)-C3H5)(HCHO)(CO)(2). [Ru(OCH2CH2CH=CH2)(CO)(3)](+) is the best for this C-C bond cleavage. This is because the coordinatively unsaturated complex with electron-accepting l igands yields a stable (eta (3)-allyl)ruthenium(II) complex in which the et a (3)-allyl ligand is strongly electron donating and needs two coordination sites. Though the C-C bond cleavage of RuBr(OCH2CH2CH=CH2)(CO)(3) occurs w ith a moderate E-a value, this reaction is much less exothermic than that o f [Ru(OCH2CH2CH=CH2)(CO)(3)](+).